Recombinant toxins are proteins made in bacteria, and they contain two parts, one for binding selectively with certain types of cells and one which kills the cell. We have been developing recombinant toxins which bind selectively to malignant hematopoietic cells, such as leukemias, lymphomas, Hodgkin's disease, and multiple myeloma. Some also bind to solid tumors such as glioblastoma and GI carcinomas. Several of the agents developed in our lab have been tested in patients with hematologic malignancies, and in each case exciting clinical remissions have been observed. Being board-certified and NCI-trained in medical oncology has enabled me to direct several of these trials at the NIH clinical center as principal investigator. This allows our research lab to become focused on relevant clinical issues for improvement of immunotoxin effectiveness, and provides excellent access to the patient samples needed for successful laboratory research studies. One clinical Phase I trial we have been conducting is with anti-Tac(Fv)-PE38 (LMB-2), a recombinant toxin which has shown promising preclinical activity and clinical responses against malignant cells displaying interleukin-2 receptors, including B- and T-cell leukemias and Hodgkin's disease. We have also initiated a phase I clinical trial of RFB4(dsFv)-PE38 (BL22) for patients with CD22+ lymphomas and leukemias. This molecule has induced complete remissions in a very high percentage of patients with chemotherapy-resistant hairy cell leukemia. In addition, a circularly permuted interleukin-4 toxin designed in the laboratory is being tested outside NIH in patients with glioblastoma multiform. A fourth molecule created in our lab, DTGM, is a fusion toxin containing human GM-CSF and truncated diphtheria toxin. DTGM is currently undergoing phase I testing outside the NIH for relapsed acute myelogenous leukemia. Our group has recently taken over the direction of NCI clinical trials of immunotoxins targeting solid tumors, such as breast, ovarian, colon, gastric, pancreatic, head & neck, and bladder cancers, as well as mesotheliomas. Our recombinant immunotoxins target mutated forms of Pseudomonas exotoxin to bind to the surface of cancer cells. After internalization, a smaller fragment is translocated to the cytosol, resulting in cell death by inhibition of protein synthesis and apoptosis. Since the toxin ADP ribosylates elongation factor 2 catalytically, one molecule is sufficient to kill a cell. The mutated toxin is fused to a binding domain, either a growth factor or a single-chain antibody, which binds selectively to cancer cells. Cells with several hundred or thousands of binding sites per cell may internalize enough so that one or several molecules reach the cytoplasm. Normal cells, which have much fewer binding sites per cell, are protected. Recombinant toxins are made by using recombinant DNA techniques to design a plasmid encoding the protein molecule, expressing the plasmid in bacteria and then purifying the protein so that it binds selectively to cancer cells. The cytotoxic activity is determined by incubating the recombinant toxins with tumor cells in tissue culture, and determining cell killing or inhibition of protein synthesis. Recombinant toxins with good cytotoxic activity are then tested in mice bearing human tumor cells, to see if the mice can be cured at safe doses. If so, the lab consults the FDA and CTEP to decide which safety studies should be done to determine if it is safe to administer the drug to people. We are uniquely positioned to test the most promising candidate agents here at NIH to determine there effectiveness in humans. A major emphasis deals with basic science questions related to intracellular metabolism and transport of toxins, and recombinant antibody engineering. Recently, the lab has begun isolating new single-chain antibodies by phage-selection after DNA-immunization of mice with the target antigen. The antigents targeted include those present on a wide range of myeloid and lymphoid leukemias, as well as Hodgkin's Disease. Recently, major emphasis has been placed on defining the etiology and prevention of the cytokine release syndrome, also called systemic inflammatory response syndrome (SIRS) which is a side effect of many recombinant toxins and monoclonal antibodies given to patients with leukemias. Methods to prevent SIRS are now being used in patients treated under our protocols. One of our most exciting questions to research involves expanded cytotoxic T-cell clones observed in patients with hairy cell leukemia. It is possible that they are an important immune response of patients against their leukemia which is blunted by the disease burden and by chemotherapy, but which is not blocked by immunotoxins. Laboratory studies should help explain how expansions of cytotoxic T-cells are related to responses in patients. Our lab has taken advantage of the potency of recombinant immunotoxins toward chemotherapy-resistant cancer cells to both successfully treat patients with these malignancies and also to utilize both clinical and basic science resources to study what these interesting proteins are doing after injection into patients. Answers to these questions should lead to improved clinical approaches both within and outside the immunotoxin field.